The invention relates to a method of processing the fluorine-containing mud produced by the acid polishing of lead glass to form basic lead carbonate therefrom.
For the improvement of surface quality, and in part also for the attainment of desired dimensions within close tolerances, lead glass products are etched in accordance with known methods with a mixture of hydrofluoric acid and sulfuric acid. In this procedure the etching acid becomes increasingly contaminated with mud.
This mud must constantly be separated from the etching acid since otherwise it might be able to cover areas of the glass products and intefere with the action of the acid, causing an irregular ablation of glass resulting in polishing flaws.
Numerous methods and apparatus are known for the separation of the mud from the etching acid. Reference might be made, for example, to German "Offenlegungschriften" Nos. 2,011,964 and 2,160,314 and German "Auslegeschriften" Nos. 1,920,009 and 2,162,815.
In the disclosures referred to above, the separation of the mud from the etching acid is described, and yet they say nothing about what can be done with the mud that is produced in large amounts.
The dumping of the mud is extremely problematical on account of its high acid content. Even if the mud is neutralized, as it is in some cases, it cannot be discarded without danger, since the soluble lead salts it contains can find their way into the ground water and thus constitute a considerable source of danger to man and animals.
A number of methods have become known which have to do with the transformation of lead compounds or mixtures.
In German Pat. No. 88,918 there is described a method of treating natural lead ores for the production of lead compounds. In this method the ores are transformed to lead sulfate with a solution containing sulfuric acid, and then further processed to lead carbonate.
German Pat. No. 886,301 discloses a process for producing a lead-rich double carbonate from substances containing lead sulfate, wherein the lead is removed by precipitation with sodium carbonate.
In German Pat. No. 936,506 there is described a process for the production of pure lead carbonate from wastes containing lead.
Lastly, a process is described in German Pat. No. 940,291 for the production of pure, lead-containing intermediate or finished products, in which lead carbonate is dissolved in acids, such as nitric acid for example, in order to separate impurities.
The substances which are treated by the above-named, known processes have one thing in common, namely that they contain neither fluorides nor fluorosilicates. The polishing mud from lead glass etching contains, in addition to lead sulfate, a high proportion of about 20 to 60% alkali hexafluorosilicates, corresponding to a fluorine content of 12 to 36%. For this reason the known processes are not suitable for the treatment of polishing mud. By the known methods of procedure it is possible at most--and this only under very unsatisfactory circumstances-- to obtain products which are greatly contaminated with fluorides and silica and therefore are not usable industrially.